[PATCH] Prevent MDNode's RAUW from introducing a reference to a void Value.

[Frédéric Riss]

> On 16 Oct 2014, at 18:58, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> On Thu, Oct 16, 2014 at 6:20 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> 
> > On Oct 16, 2014, at 6:08 PM, Eric Christopher <echristo at gmail.com <mailto:echristo at gmail.com>> wrote:
> >
> > On Thu, Oct 16, 2014 at 6:06 PM, Duncan P. N. Exon Smith
> > <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
> >>
> >>> On Oct 16, 2014, at 3:54 PM, Frederic Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
> >>>
> >>> If you look at the added testcase, it cast a void (*func)() to a int (*func)(). The int typed result is referenced in a debug info metadata node (an argument to the dbg.value intrinsic). Then instcombine would remove the bitcast and turn the int returning function into a void returning function that gets RAUWd over the value in the MDNode. And at this point in the current code you have invalid IR.
> >>>
> >>> The code in the testcase is reduced from a huge LTO link that encountered this situation in a much more complicated setup.
> >>>
> >>> http://reviews.llvm.org/D5828 <http://reviews.llvm.org/D5828>
> >>
> >> Weird.  This LGTM (after fixing a few super minor whitespace changes);
> >> not sure if Eric is still looking.
> >
> > Enh, I was trying to figure out if there was some way to avoid the
> > void metadata node being create rather than avoiding RAUW'ing it. It's
> > probably not a huge distinction at the moment.
> 
> I don't think it is a void metadata node; it's a void `CallInst`.
> 
> We have:
> 
>     call void @llvm.dbg.declare(metadata !{i32* %deadvar}, ...)
>     %call = ...
>     store i32 %call, i32* %deadvar, align 4, !dbg !21
> 
> Which converts to:
> 
>     call void @llvm.dbg.value(metadata !{i32* %call}, ...)
>     %call = ...
> 
> Then, when %call gets RAUW'ed, `metadata !{i32* %call}` gets updated
> to `void`.
> 
> My only thought is that this code has UB, right? I don't know whether we should do something else with it, which might involve killing the dbg.value metadata along with all the other instructions that are fruit of the UB tree. (the whole basic block, anything else reachable from it, etc)

Are you advocating against the patch, or merely pointing out that the compiler should handle the testcase code differently? In the former case, here’s my answer :-) :

Yes, it involves UB (at least I think, the bogus return value is never used, but I suppose the call through the wrong pointer type in itself is UB). And thus we would be in our right to just drop the call and the associated dbg.value, but the fix in itself has nothing to do with UB. It prevents any call of RAUW with a void type replacement Value to generate invalid MDNodes. We could audit all RAUV callers to never do that, but handling it directly in MDNode seems safer (and what would the caller do anyway? the end-result would certainly be the same).

The counter argument could be that only input code with UB could trigger such RAUW calls, but I wouldn’t bet on it.

Fred


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